Temperatures and performance: the human body, a machine to adapt.
Humans have adaptive biological mechanisms and behavioral responses to cope with environmental stresses. Typically, adaptation occurs after repeated exposures to specific stresses. However, these mechanisms vary between "normal" athletes and those with cardiomyopathy. Individuals with pre-existing medical conditions are at higher risk of cardiovascular events when faced with certain stresses, with environmental factors playing a role in both adaptation and the occurrence of cardiac incidents.
Outdoor sports activities, from a few seconds to ultra-marathons lasting over 24 hours, illustrate the complexity and importance of research into cardiovascular adaptation mechanisms to diverse environments. These mechanisms include adjustments in heart rate (HR), blood pressure (BP), and cardiac output (CO), essential for maintaining performance under varied conditions (see Diagram above).
It appears that the type of sport and the training program are the main factors in cardiac adaptation in athletes. How do our bodies adapt? What effects does temperature have on our endurance performance?
The effects of temperature on training.
The interaction between cold and exercise poses unique challenges. Recent studies show that exercise in a moderately cold environment, compared to a thermoneutral climate, can benefit the cardiovascular system by increasing the body's tolerance to stress and promoting cardiovascular health. The underlying mechanisms include the prevention of excessive core temperature rise, activation of the neuro-immuno-endocrine network, triggering of the antioxidant system, bioenergetic and metabolic reconfiguration, as well as the secretion of various exerkines (molecules released in response to exercise, which have the potential to improve cardiovascular, metabolic, immunological, and neurological health). Additionally, peripheral vasoconstriction, which increases central venous pressure by shifting blood from peripheral to central circulation, leads to an increase in stroke volume (SV) and a rise in VO2.
Intense endurance exercise generates significant heat, facilitating the maintenance of optimal body temperature in a slightly cool environment. Aerobic performance is optimal at an ambient temperature between 10 and 12°C, for example for marathons. However, beyond this temperature, performance decreases exponentially, although other factors, such as heat exchange, exercise type, and climatic conditions (wind, humidity, radiation) also play a role.
Risks in case of extreme temperatures:
Humans possess remarkable thermoregulatory capacities, allowing adaptation to various temperature changes, particularly in cold climates. However, exposure to extremely cold environments, such as cold air or water, can disrupt thermoregulation, unbalancing metabolic heat production and dissipation, and potentially causing hypothermia (core temperature < 35 °C). For example, a drop in core temperature of 0.5 to 2.0 °C increases resting metabolism and reduces cardiac output (CO) and VO2 max.
When skin temperature drops below 35°C, heat loss decreases, leading to rapid cooling of exposed areas such as the face, fingers, and toes, which can impair manual dexterity and tactile sensitivity. However, periodic episodes of vasodilation can prevent excessive vasoconstriction and reduce the risk of cold injuries. In contrast, shivering – rhythmic contractions of skeletal muscles – can increase metabolism up to 46% of VO2 max, representing a sevenfold increase in resting metabolism.
The risk of hypothermia is significantly increased when immersed in cold water, as heat loss by convection is about 20 times higher than in air. In these cases, the cold shock response can be triggered, including involuntary inspiration, followed by uncontrollable hyperventilation, tachycardia, and increased release of stress hormones.
Factors that evolve with practices.
Another external factor is wind, whose effect varies with temperature, speed, direction, and athlete movement. While favorable in some sports, wind accelerates heat loss in cold environments, which is not always desirable. In high-altitude mountaineering, wind combined with wet clothing quickly replaces the humid air layer with drier air, increasing sweat evaporation and cooling. Sports like running and skiing also create airflow, accentuating the effect of cold. Notably, wind increases heat loss by convection, further exposing to frostbite, which occurs when skin and tissues are subjected to temperatures below 0 °C.
Increased altitude is correlated with a decrease in the partial pressure of oxygen in the air, so the amount of oxygen available to muscles and organs is reduced. This can lead to a short-term decrease in aerobic performance and a faster sensation of fatigue. Over time, the body will adapt in three ways: increased red blood cell production (which transports oxygen) via erythropoietin, increased ventilation, and increased mitochondrial efficiency.
Finally, in high-speed sports (e.g., cycling), airflow facilitates heat dissipation, while higher ambient temperatures reduce air density and aerodynamic drag. This means that high heat and humidity may impact these sports less than slower activities, such as athletics, for example.
Conclusion: Adaptation, but at what cost?
The human body, a true adaptation machine, is capable of overcoming extreme environmental conditions through complex physiological mechanisms and behavioral adjustments. These responses, although remarkable, are not infallible and vary depending on the individual, the activity practiced, and the conditions encountered. Training in specific environments, whether cold, hot, humid, or at altitude, pushes the body to its limits while revealing its adaptive capacities, such as improved cardiac efficiency or metabolic optimization.
However, these adaptations come at a cost: prolonged or poorly managed exposure can lead to health risks, particularly cardiovascular ones. Understanding the underlying mechanisms and their interactions with external factors such as temperature, wind, or altitude is essential to maximize benefits while minimizing dangers.
Scientific research continues to explore these processes to better guide athletes in their pursuit of performance, while ensuring their safety in the face of environmental challenges. Beyond performance, these discoveries pave the way for a better understanding of human resilience in extreme conditions, a fascinating field where adaptation, survival, and self-transcendence intertwine.
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